We want to create 40-50 custom fields in the opportunity and case module. Is there any issue to have this?
You won't have any issue doing it, you may have an issue getting your users to fill that many fields though...
Too often I've seen management wanting all these metrics and requesting a whole lot of fields be added but then it all becomes too cumbersome for the user and no one takes the time to fill them. Just something to think about. Personally, I would question the business need for so many fields.